Contact structure



. 1967 c. J. BISHOFBERGER 3,304,467

CONTACT STRUCTURE Filed Jan. 8, 1965 z7 g 11 15 25, PRlMARY SECONDARY 65' m VILNIOR. 64 MOTOR 6521a: =7. Brsworazzazk United States Patent Office 3,3MA67 CONTACT @TRUCTURE Carl .l. Bishofherger, Minneapolis, Minn, assignor to Honeywell Inc., a corporation of Delaware Filed Jan. 8, 1965, Ser. No. 424,368 8 Claims. (Cl. 31715) The present invention is directed to an electrical contact structure, and more particularly is directed to a contact structure that is adapted to fail while carrying current in cyclic operation.

In the art of manufacturing or using ordinary electrical contact structures, longevity is normally one of the major concerns. Ordinarily electrical contact structures are designed for thousands of operations without possible failure or welding of the contact members. The use of such contact structures involves the incorporation of relatively precious metals and involves a substantial cost. In certain types of applications where long life contacts would provide a satisfactory operational function, their cost becomes rather severe due to the amounts of precious contact materials that are required to obtain the necessary life. In certain types of applications a long life type of contact structure, while providing an acceptable function, may be better served by the utilization of a contact structure that has the inability to operate tfOl' more than a relatively short period of time before failing in a specific manner. This type of structure is in a current overload device wherein a cyclic type of operation is utilized to prevent an overload current from damaging some protected device.

More specifically, if a bimetal overload device is provided in series with a transformer secondary, large contacts must be provided to provide long cycling life so that the transformer will meet the requirements of industry as defined by the standards in use by Underwriters Laboratories. At the present time, a current overload device in the secondary circuit of a control transformer has the requirement that under overload conditions that the bimetal operating mechanism and contacts must be capable of cycling for a fifteen day period and still be operative at the end of this time period. In the utilization of a transformer under overload conditions, cycling may occur at a rate of a cycle every few seconds and thereby in a fifteen day period run into the thousands of cycles. In order to provide a protective device that would meet this standard, very large and expensive contact members are required. In order to overcome this and provide a safety device of the cycling type which has a more desirable type of function, the present invention evolved.

Another type of overload protection that has been acceptable in industry is a fuse type of protection or a one shot type of device. The one shot type of device utilizes some type of fusible metal that melts when an overload current exists. This arrangement removes the transformer from the circuit but has a deficiency in that no leeway is allowed in the installation for locating a minor trouble before the transformer secondary is removed from the circuit on a permanent basis. Since small transformers usually do not utilize a replacement fusible element, the entire transformer requires replacement if the overload protection of the one shot type is utilized and a short term overload occurs.

The present invention is directed to a new type of overload protection utilizing a contact structure that fails under cyclic operation. More particularly, the present invention utilizes a pair of contact members that complete an electric circuit in series with the normal transformer secondary and which operate cyclicly to interrupt the current fiow under overload conditions. One of the con- 3,304,467 Patented Feb. 14, 1967 tact elements is a normal contact in structure, while the other contact element is an insulating support member which includes a thin film of contact material which completes the electric circuit through the contact element itself. As the cycling action occurs, a certain amount of the contact material erodes and eventually the thin film of contact material on the insulating member is eroded away until an open circuit exists in the contact member. The use of the thin film of contact material assures that the contact surfaces cannot weld to one another as the thin film cannot mechanically support a Weld.

It has been found that with a 40 volt-ampere type transformer having a normal 25 volt open circuit voltage, that a two mil thickness of silver coated on an insulating member and operated by a bimetal will provide a cycling action to interrupt a short circuit secondary current for approximately one-half hour before a hole erodes through the coating of the thin film of contact material thereby open circuiting the secondary of the transformer. If longer cycling times are required, the thickness of the film can be increased. A film of approximately five mils in thickness operated by the same type of transformer with a short circuit load will cycle for approximately twelve hours before the open'circuit condition drops the transformer secondary out of the circuit by open circuiting across the contact structure.

The utilization of the present invention leads to two distinct advantages. By providing a transformer that has overload protection that cycles for a relatively short period of time, such as a half-hour, an installer has more than adequate time to locate a fault that would ordinarily blow out the overload element in a one shot type of protection and thereby require the installation of an entirely new transformer. The second major advantage is to provide a cycling device that drops out of the circuit in a relatively short period of time thereby shutting down the system and indicating that some trouble requires correction in a shorter period of time than is allowed under the Underwriters Laboratory requirement for a cycling device.

The present invention can be more clearly understood when the present specification is considered with the detailed drawings, wherein:

FIGURE 1 is a schematic representation of a transformer circuit incorporating the novel contact structure in the form of an overload;

FIGURE 2 is an elevation of a standard type of control transformer;

FIGURE 3 is an end view of the transformer of FIG- URE 2 taken along line 3-3;

FIGURE 4 is a cross section of the element of FIG- URE 3 along line 44;

FIGURE 5 is an enlarged cross section of the contact structure of FIGURE 4; and

FIGURE 6 is a semi-schematic representation of the contact structure incorporated in a timing mechanism.

In FIGURE 1 a transformer 10 is generally disclosed having a primary winding 11, a core 12 and a secondary winding 13. The primary winding 11 is terminated in a pair of leads 14 and 15 in a conventional manner. The secondary winding 13 is terminated in a conventional lead 16 and a wire 17 that is connected to an overload protection means disclosed at 20. The overload protection means 20 incorporates the novel contact structure of the present invention and is disclosed in detail in the FIGURES 2 through 5. The overload protection means 20 generally includes a bimetal element 21 and a pair of contacting elements 22 and 23. The function of the last named elements will be amplified below.

In FIGURE 2 an elevation is disclosed of a standard control transformer 10 that is schematically represented in FIGURE 1. The transformer 10 discloses a mounting :plate 24 along with end covers 25 and 26 that are supported on the core 12. The conductors 14 and 15 are disclosed as passing through the end cover 26 while the end cover 25 is disclosed supporting an insulating support member 27 by tabs 28 and will be described in detail in connection with FIGURE 3.

In FIGURE 3 the insulating support 27 is disclosed mounted on the housing 25 and held in place by the tabs 28. The insulating support 27 mounts a pair of screw type terminals 31) and 31 with conductor 16 electrically attached to the terminal 31 to complete one side of a junction to the secondary 13 of the transformer 10. Attached to the terminal 30 is a contact support 32 and the contact 23 that both fall below the insulating support member 27, as clearly shown in FIGURE 4. The contact 23 is of any good electrical contact material such as silver, platinum, gold, or alloys of various other materials Well known in the electrical contact art. The contact 23 can either be solidly made of this material or be mere-1y coated with an adequate covering of this material as is customarily done in the manufacture of electrical contacts. In some cases the contact 23 could be made of any type of electrically conductive material as will become apparent when the function of the contact structure is fully disclosed.

Attached to the insulating support member 27 by a rivet 34 is the bimetal element 21. A projection 35 is provided on thebimetal 21 and projects through a hole 36 in the insulating support 27. This hole is used for alignment purposes as well as a means for the passage of the wire 17 for connection, as is seen in FIGURE 3, to the bimetal 21 at projection 35. By this means, the sec ondary winding 13 is appropriately connected by the wire 17 to the bimetal projection 35 thereby supplying a circuit to the bimetal 21 forthe purposes described below. An end 38 of the bimetal 21 is connected by rivet 37 to the contact member 22. The contact member 22 in reality is formed of an insulating material 40' covered with a thin film of contact material 41. Once again the contact material 41 can be any conventional contact material or good conductor as Will become evident later in present description. It is thus apparent that a circuit is completed between the bimetal 21, the rivet 37, the film 41, and the contact 23 between the projection 35 of the bimetal 21 and the terminal 30. The bimetal 21, the rivet 37, the insulating support 40 and the conductive film 41 are shown in greatly enlarged cross-section in FIGURE for clarity sake.

OPERATION In normal operation, the transformer disclosed in FIG- URES 1 and 2 would have the primary winding 11 energized from some conventional source of electric power, such as 120 volts, 60 cycles. The secondary winding 13 would have a normal open circuit output voltage of 25 volts, 60 cycles, that would be available between the conductors 16 and the terminal 30, or in FIGURE 3 between the terminals 30 and 31. Interpose-d in the secondary is the bimetal overload protection means 20 that provides a normally closed circuit thereby supplying power to the terminals 30* and 31. The terminals 30 and 31 would be connected to any type of control circuit where the lower nominal voltage was desired. Under the normal current drain conditions through the transformer secondary 13, the heating of the current flowing through the bimetal 21 would not affect the position of the bimetal thereby leaving it in the position shown in FIG- URE 4.

Upon the existence of a short circuit or a substantial overload on the secondary 13 of transformer 10, the current in the secondary winding 13 increases sharply. This increase in current flow passes through the bimetal 21 and the internal resistance of the bimetal 21 heats the bimetal and warps the bimetal in a downward direction from that shown in FIGURE 4. The warpage eventually open circuits the current overload protection means 20 by moving the member 22 in a downward direction thereby separating the contact 23 and the thin film of contact material 41. The bimetal heating operation under overload and the movement of two contact surfaces from one another under these conditions is a well known expedient. Also it is well known that when this occurs a certain amount of the contact material erodes due to the are that is drawn Whenever an electric circuit is interrupted. Under overload conditions, the amount of eroding is aggravated and some small amount of the conductive fi-lm 41 is transferred or eroded away. As soon as the circuit through the overload means 20 is opened, the bimetal 21 begins to cool and returns to the closed position shown in FIGURE 4. The condition of overload, still existing, causes the bimetal 21 to in turn heat once again and cyclic operation of the contact structure 22 is begun. Actual tests on a device of this type indicate that the device cycles quite rapidly thereby progressively eroding more and more of the conductive film 41. The cycling continues until the film 41 is eroded to the point where the contact 23 no longer meets with the film 41, but is stopped by the insulating support 41). At this time there is no longer an electric circuit between the terminal 30 and the bimetal 21 thereby open circuiting the secondary 13 of the transformer 10.

It can thus be seen that by providing a cyclic type of device wherein a thin film of contact material is provided on an insulating base, that an eroding action can be utilized to eventually open circuit the cyclic device. The thin film of contact material is selected so that the two contact surfaces cannot weld tog-ether and that the failure is always as an open circuit. As was previously stated, tests have indicated that the two mil thickness of contact material at 41 provide a cycling time of approximately one-half hour under short circuit conditions for a 40 voltampere transformer. It is thus apparent that an accidental overload on the transformer or an overload which is promptly located and removed would not require the replacement of the entire transformer as long as the fault was located within a tone-half hour period. This provides for more than adequate safety in the installation of control transformer and also provides that when the control transformer has cycled for a sufiiciently long period of time under overload conditions, the overload is removed entirely from the circuit by an open circuit condition in a safe and convenient manner. Due to the fact that the amounts of contact material utilized are very small due to the thinness of the film 41 and the small amount of contact material 23, the cost of the present overload device is substantially below the cost of the more conventional bimetal overload utilizing heavy duty contacts that are capable of cycling continuously for 15 or more days as is required by the Underwriters Laboratories. It is thus apparent that a better, and more economical device is disclosed than has previously been disclosed in the overload protection art.

The novel contact structure disclosed above had additional utility other than as an overload protection in a transformer secondary. An additional use for this type of contact structure is disclosed in FIGURE 6. In FIG- URE 6 an insulating support 50 is provided with a terminal 51 mounted thereon and having an end 52 passing through the support 50. Also mounted on the insulating support 50 is a thin film of contact material 53 which is directly applied to the insulating member 50 or is held on the insulating member 50 by the end 52 of the terminal 51. The insulating member 50 also has a second terminal 54 that passes through the insulating support 50 and has an end 55 that mounts a flexible lever 56 on the support 50. The end of the lever 56 has a contact 57. The terminal 51 is electrically connected to the film 53 which in turn makes direct contact with the contact member 57 and electrical contact with the member 56 back to the terminal 54. In the position shown it is obvious that a short circuit exists between the terminals 51 and 54 through the structure just described. A pair of conductors 60 and 61 are connected to the terminals 51 and 54 to be provided as a connection means to a circuit for a purpose which will be described later. A mechanical linkage or drive 62 is connected at a pivot 63 on an eccentric 64 which is driven at 65 by a motor shaft 66.

The motor shaft. 66 is in turn driven by the motor 67 which receives power from conductors '70 and 71.

OPERATION OF FIGURE 6 The device disclosed in FIGURE 6 could be used, for example as a simple timing device. Upon applying power tothe conductors 70 and 71 the motor 67 turns the shaft 66 that in turn operates the eccentric mechanism 64 so that the linkage 62 is reciprocated as the eccentric 64 rotates. The reciprocation of the linkage 62 moves the member 56 in an up and down direction thereby repeatedly opening and closing the contact 57 against the film 53. It is thus obvious that if an electric circuit is connected between the conductors 60 and 61, that the circuit is repeatedly broken until such time as the thin film 53 is eroded to open circuit the conductive equipment between the terminals 51 and 54. The device disclosed in FIGURE 6 could be used as an overload device or a timing device and its application would vary with need and would be obvious to one skilled in the present art.

The applicant has disclosed a very unusual contact structure that utilizes a contact means including a contact and a second contact element having a thin film of contact material supported by an insulating member wherein the film is eroded away to open circuit the contact means upon cyclic operation under load and overload conditions. The present invention has been shown and incorporated both as an overload device and as a timing device, and it becomes obvious that its application could be widely used in many types of devices. As a result of this, the applicant wishes to be limited in the scope of his invention only by the scope of the appended claims.

I claim as my invention:

1. An electrical contact structure adapted to fail under cyclic operation, including: cyclically movable means for operation of contact means; and said contact means having an insulating member including a thin film of contact material cooperating with an electrical contact to complete an electric circuit through said contact means; said cyclically movable means repeatedly moving said insulating member with respect to said electrical contact to interrupt a current drawn through said contact means until said thin film of contact material is eroded to open circuit said contact means.

2. An electrical contact structure adapted to fail under cyclic operation, including: current source means; electrically energized cyclically movable means including connection means connected to said current source means to provide a circuit for a current drawn from said current source; and contact means operated by said movable means and having an insulating member including a thin film of contact material cooperating with an electrical contact to complete an electrical circuit for said current through said contact means; said cyclically movable means moving said insulating member with respect to said electrical contact to interrupt the current drawn through said contact means from said current source means until said thin film of contact material is eroded to open circuit said contact means.

3. A current overload protection device, including: a bimetal element fixed at a first end and free to move at a second end; said movable end operating contact means; and said contact means having an insulating member including a thin film of contact material cooperating with an electrical contact to complete an electric circuit through said device; said current overload protection device being adapted to be connected in a series electrical circuit wherein said overload device cyclically interrupts an overload current drawn through said contact means until said thin film of contact material is eroded to open circuit said contact means.

4-. A current overload protection device, including: a bimetal element fixed at a first end and free to move at a second end; said movable end operating contact means; and said contact means having an insulating member including a thin film of contact material attached to said second end of said bimetal element to cooperate with a fixed electrical contact to complete an electric circuit through said device; said current overload protection device being adapted to be connected in a series electrical circuit wherein said overload device cyclically interrupts an overload current drawn through said bimetal element and said contact means until said thin film of contact material is eroded to open circuit said contact means.

5. An electric power supply means, including: current source means; bimetal overload protection means including a bimetal element fixed at a first end and free to move at a second end; said movable end operating contact means with said contact means having an insulating member including a thin film of contact material cooperating with an electrical contact surface to complete an electric circuit through said overload protection means; and circuit means connecting said current source means and said bimetal overload protection means in a series electrical circuit with said overload means cyclically interrupting an overload current drawn from said current source means until said thin film of contact material is eroded to open circuit said overload protection means.

6. An electric power supply means, including: current source means and bimetal overload protection means connected in a series electrical circuit with said overload means cyclically interrupting an overload current drawn from said current source means; said bimetal overload protection means including a bimetal element fixed to support means at a first end and free to move at a second end; said support means and said second movable end supporting contact means; and said contact means including an insulating member having a thin film of contact material cooperating with an electrical contact surface to complete an electric circuit through said overload protection means when a normal current is being drawn from said current source means; said overload protection means cyclically interrupting said overload current until said thin film of contact material is eroded to open circuit said overload protection means.

7. An electric power supply means, including: a transformer having a primary winding and a secondary winding with said primary winding connected to a source of electrical energy; bimetal overload protection means including support means mounted upon said transformer; said overload protection means including a bimetal element fixed to said support means at a first end and free to move at a second end; said movable end operating contact means with said contact means having an insulating member including a thin film of contact material cooperating with an electrical contact to complete an electric circuit through said overload protection means; and circuit means connecting said transformer secondary winding and said bimetal overload protection means in a series electrical circuit with said overload means cyclically interrupting an overload current drawn from said transformer until said thin film of contact material is eroded to open said overload protection means.

8. An electric power supply means, including: a transformer having a primary winding and a secondary winding with said primary winding connected to a source of electrical energy; bimetal overload protection means including support means mounted upon said transformer; said overload protection means including a bimetal element fixed to said support means at a first end and free to move at a second end; said movable end operating contact means with said contact means having an insulat- 7 v 8 ing member including a thin film of contact material at- References Cited by the Examiner tached to said second end of said bimetal element to 00- UNITED STATES PATENTS operate with a fixed electrical contact to complete an electric circuit through said overload protection means; and 2,074,559 3/1937 Radtke 200 f113 circuit means connecting said transformer secondary 5 FOREIGN PATENTS winding and said bimetal overload protection means in a series electrical circuit with said overload means cyclically interrupting an overload current drawn from said trans- MILTON O HIRSHFIELD Primary Examiner, form-er until said thin film of contact material is eroded to open said overload protection means. 10 TRAMMELL, AS81810"! Examiner:

270,969 5/1927 Great Britain. 

1. AN ELECTRICAL CONTACT STRUCTURE ADAPTED TO FAIL UNDER CYCLIC OPERATION, INCLUDING: CYCLICALLY MOVABLE MEANS FOR OPERATION OF CONTACT MEANS; AND SAID CONTACT MEANS HAVING AN INSULATING MEMBER INCLUDING A THIN FILM OF CONTACT MATERIAL COOPERATING WITH AN ELECTRICAL CONTACT TO COMPLETE AN ELECTRIC CIRCUIT THROUGH SAID CONTACT MEANS; SAID CYCLICALLY MOVABLE MEANS REPEATEDLY MOVING SAID INSULATING MEMBER WITH RESPECT TO SAID ELECTRICAL CONTACT TO INTERRUPT A CURRENT DRAWN THROUGH SAID CONTACT MEANS UNTIL SAID THING FILM OF CONTACT MATERIAL IS ERODED TO OPEN CIRCUIT SAID CONTACT MEANS. 